A variety of granular particles are widely used as propping agents to maintain permeability in oil and gas formations. Three grades of proppants are conventionally employed: sand, resin-coated sand, and ceramic proppants. Conventional proppants exhibit exceptional crush strength but also extreme density. A typical density of ceramic proppants exceeds 100 pounds per cubic foot. Proppants when used in oil and gas wells are materials pumped into oil or gas wells at extreme pressure in a carrier solution (typically brine) during the hydrofracturing process. Once the pumping-induced pressure is removed, proppants “prop” open fractures in the rock formation and thus preclude the fracture from closing. As a result, the amount of formation surface area exposed to the well bore is increased, enhancing recovery rates. Proppants also add mechanical strength to the formation and thus help maintain flow rates over time. Proppants are principally used in gas wells, but do find applications in oil wells.
Relevant quality parameters, especially when proppants are used to enhance oil or gas production, include: particle density (low density is desirable), crush strength and hardness, particle size (value depends on formation type), particle size distribution (tight distributions are desirable), particle shape (spherical shape is desired), pore size (value depends on formation type and particle size, generally smaller is better), pore size distribution (tight distributions are desirable), surface smoothness, corrosion resistance, temperature stability, and hydrophilicity (hydro-neutral to phobic is desired). Lighter specific gravity proppants can be desirable, since they are easier to transport in the fracturing fluid and therefore can be carried farther into the fracture before settling out and which can yield a wider propped fracture than higher specific-gravity proppants.
Proppants used in the oil and gas industry are often sand and man-made ceramics. Sand is low cost and a moderate density compared to other proppant materials, but has low strength. Man-made ceramics, mainly bauxite-based ceramics or mullite-based ceramics, are much stronger than sand, but are far more dense and more costly than sand. Ceramic proppants dominate sand and resin-coated sand on the critical dimensions of crush strength and hardness. They also offer some benefit in terms of maximum achievable particle size, corrosion, and temperature capability. Extensive theoretical modeling and practical case experience suggest that conventional ceramic proppants offer compelling benefits relative to sand or resin-coated sand for most formations. Ceramic-driven flow rate and recovery improvements of 20% or more relative to conventional sand solutions are not uncommon.
Current ceramics proppants are typically employed in wells of intermediate to deep depth. Shallow wells typically employ sand or use no proppants. Ceramic proppants were initially developed for use in deep wells (e.g., those deeper than 7,500 feet) where sand's crush strength is inadequate. In an attempt to expand their addressable market, ceramic proppant manufacturers have introduced products focused on wells of intermediate depth.
Resin-coated sands offer a number of advantages relative to conventional sand. First, resin coated sand exhibits higher crush strength than uncoated sand given that resin-coating disperses loads over a wider area, reducing stresses within the proppant. Second, resin-coated sands are “tacky” and thus exhibit reduced “proppant flow-back” relative to conventional sand proppants. In other words, the resin-coated sand proppant is more likely to remain in the formation. Third, the resin coating typically increases sphericity and roundness of the proppant, thereby reducing flow resistance through the proppant pack.
Recent developments in ceramic proppants have sought to maintain crush strength while reducing proppant density. As an example, porosity has been introduced into proppant bodies. The introduction of pores into proppant bodies have generally corresponded with reduced strength.
In each application of proppants to an oil or gas formation, some proppants are crushed. Porous ceramic proppants tend to generate significant amounts of fine particles which can be carried from the formation. Fines must be filtered and can abrade the equipment used during well production.
Ceramic proppants are typically formed using a standard series of processes:
1) Mix green body materials
2) Form green-body shape
3) Sinter green-body into final ceramic proppant.
Traditional processes to produce ceramic particles such as proppants are time consuming and costly. Raw materials are typically pre-ground to size. Then the sized materials are transferred to a mixer having medium to intensive shear in order to form a uniform dispersion or green body material. The green body material is then formed into particles by another process such as spray drying, compaction, or milling. The formation process may be repeated several times if a multi-layer ceramic particle configuration is desired. The stable, unsintered particle that is formed whether in a single or multi-layered process is called a “green body.” Finally, the green body is sintered to produce a finished ceramic, glass-ceramic, or composite. The multiple steps in this process require many pieces of equipment, a large manufacturing area, and the need to transfer material from one piece of equipment to the next. Most of the steps in the traditional process are batch operations.
Extrusion processes are well known as methods to form ceramics. U.S. Pat. No. 3,112,184 describes a method to make thin-walled ceramic honeycomb structures for use in regenerators, recuperators, radiators, catalyst carriers, filters, heat exchangers, and the like. Such ceramic extrusions are useful for producing large, structural articles. U.S. Pat. No. 5,227,342 describes a method for making metal oxide ceramics, such as pellets or plugs, by an extrusion process. U.S. Pat. No. 7,160,584 describes a method for manufacturing a ceramic glow pin that is formed of more than two layers and manufactured by a co-extrusion process. Such ceramic extrusions can produce smaller shapes, such as pellets. However, these methods are not useful for articles, such as proppants, because proppants must be generally spherical in shape. When bisected by three mutually perpendicular planes, a sphere has a circular cross section in each plane. Extrusions can produce spherical cross sections in only one plane perpendicular to the direction of the extrudate as it exits the die. Furthermore, proppants with multiple layers must enclose or encapsulate the inner layers in the generally spherical proppant. In co-extrusions, multiple, adjacent layers, or regions are extruded simultaneously. If the extrudate from a coextrusion was cut perpendicular to the flow of material, inner layers or regions of material would not be completely encapsulated as all materials present in the coextrusion are visible in the extrudate when viewed in the direction perpendicular to the extruder die. Pellets cut from a coextrusion expose the inner layers of the structure at the surface where the pellet is cut.